James E. Owen

Planetary evaporation by UV and X-ray radiation: basic hydrodynamics

We consider the evaporation of close-in planets by the stars intrinsic extreme-ultraviolet (EUV) and X-ray radiation. We calculate evaporation rates by solving the hydrodynamical problem for planetary evaporation including heating from both X-ray and EUV radiation. We show that most close-in planets (a < 0.1 au) are evaporating hydrodynamically, with the evaporation occurring in two distinct regimes: X-ray driven, in which the X-ray heated flow contains a sonic point, and EUV driven, in which the X-ray region is entirely sub-sonic. The mass-loss rates scale as LX/a2 for X-ray driven evaporation, and as for EUV driven evaporation at early times, with mass-loss rates of the order of 1010–1014 g s−1. No exact scaling exists for the mass-loss rate with planet mass and planet radius; however, in general evaporation proceeds more rapidly for planets with lower densities and higher masses. Furthermore, we find that in general the transition from X-ray driven to EUV driven evaporation occurs at lower X-ray luminosities for planets closer to their parent stars and for planets with lower densities. Coupling our evaporation models to the evolution of the high-energy radiation – which falls with time – we are able to follow the evolution of evaporating planets. We find that most planets start off evaporating in the X-ray driven regime, but switch to EUV driven once the X-ray luminosity falls below a critical value. The evolution models suggest that while ‘hot Jupiters’ are evaporating, they are not evaporating at a rate sufficient to remove the entire gaseous envelope on Gyr time-scales. However, we do find that close in Neptune mass planets are more susceptible to complete evaporation of their envelopes. Thus we conclude that planetary evaporation is more important for lower mass planets, particularly those in the ‘hot Neptune’/‘super Earth’ regime.

Theoretical spectra of photoevaporating protoplanetary discs: an atlas of atomic and low-ionization emission lines

We present a calculation of the atomic and low-ionization emission line spectra of photoevaporating protoplanetary discs. Line luminosities and profiles are obtained from detailed photoionization calculations of the disc and wind structures surrounding young active solartype stars. The disc and wind density and velocity fields were obtained from the recently developed radiation-hydrodynamic models of Owen et al. that include stellar X-ray and EUV irradiation of protoplanetary discs at various stages of clearing, from primordial sources to inner hole sources of various hole sizes. Our models compare favourably with currently available observations, lending support to an X-ray driven photoevaporation model for disc dispersal. In particular, we find that X-rays drive a warm, predominantly neutral flow where the O I 6300 A line can be produced by neutral hydrogen collisional excitation. Our models can, for the first time, provide a very good match to both luminosities and profiles of the low-velocity component of the O I 6300 A line and other forbidden lines observed by Hartigan et al., which covered a large sample of T-Tauri stars. We find that the O I 6300 A and the Ne II 12.8 μm lines are predominantly produced in the X-ray-driven wind and thus appear blueshifted by a few km s −1 for some of the systems when observed at non-edge-on inclinations. We note, however, that blueshifts are only produced under certain conditions: X-ray luminosity, spectral shape and inner hole size all affect the location of the emitting region and the physical conditions in the wind. We caution therefore that while a blueshifted line is a tell-tale sign of an outflow, the lack of a blueshift should not be necessarily interpreted as a lack of outflow. Comparison of spectrally resolved observations of multiple emission lines with detailed model sets, like the one presented here, should provide useful diagnostics of the clearing of gaseous discs.

Two populations of transition discs

We examine the distribution of transition discs as a function of millimetre (mm) flux. We confirm that as expected in any model in which most primordial discs turn into transition discs and in which mm flux declines with time, transition discs have lower mm fluxes on average than primordial discs. However, we find that the incidence of transition discs does not, as expected, fall monotonically towards large mm fluxes and also investigate the hypothesis that these mm bright transition discs may have a distinct physical origin. We find that mm bright transition discs occupy a separate region of parameter space. Transition discs in the bright mm subsample have systematically higher accretion rates than those in the faint mm subsample, along with being systematically weighted to earlier spectral types.

Water loss from terrestrial planets orbiting ultracool dwarfs: implications for the planets of TRAPPIST-1

Ultracool dwarfs (UCD) encompass the population of extremely low mass stars (later than M6-type) and brown dwarfs. Because UCDs cool monotonically, their habitable zone (HZ) sweeps inward in time. Assuming they possess water, planets found in the HZ of UCDs have experienced a runaway greenhouse phase too hot for liquid water prior to entering the HZ. It has been proposed that such planets are desiccated by this hot early phase and enter the HZ as dry, inhospitable worlds. Here we model the water loss during this pre-HZ hot phase taking into account recent upper limits on the XUV emission of UCDs and using 1D radiation-hydrodynamic simulations. We address the whole range of UCDs but also focus on the planets b, c and d recently found around the 0:08 M dwarf TRAPPIST-1. Despite assumptions maximizing the FUV-photolysis of water and the XUV-driven escape of hydrogen, we find that planets can retain significant amounts of water in the HZ of UCDs, with a sweet spot in the 0:04 ‐ 0:06 M range. With our assumptions, TRAPPIST-1b and c can lose as much as 4 Earth Ocean but planet d ‐ which may be inside the HZ depending on its actual period ‐ may have kept enough water to remain habitable depending on its initial content. TRAPPIST-1 planets are key targets for atmospheric characterization and could provide strong constraints on the water erosion around UCDs.

Magnetically controlled mass-loss from extrasolar planets in close orbits

We consider the role magnetic fields play in guiding and controlling mass-loss via evaporative outflows from exoplanets that experience UV irradiation. First we present analytic results that account for planetary and stellar magnetic fields, along with mass-loss from both the star and planet. We then conduct series of numerical simulations for gas giant planets, and vary the planetary field strength, background stellar field strength, UV heating flux, and planet mass. These simulations show that the flow is magnetically controlled for moderate field strengths and even the highest UV fluxes, i.e., planetary surface fields

The interplay between X-ray photoevaporation and planet formation

B_P\gtrsim 0.3

The Origin and Evolution of Transition Discs: Successes, Problems, and Open Questions

gauss and fluxes

The imprint of photoevaporation on edge-on discs

F_{UV}\sim10^{6}

An asteroseismic view of the radius valley: stripped cores, not born rocky

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The Ṁ–M* relation of pre-main-sequence stars: a consequence of X-ray driven disc evolution

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